Crude oil-derived kerosene-gas oil distillate fractions generally contain sulfur compounds. When these oils are used as a fuel for diesel-powered vehicles, sulfur which exists in the sulfur compounds is converted to low molecular weight sulfur compounds, and exhausted in the air. Further, in an exhaust gas treatment apparatus which is recently being introduced, when the sulfur compound exists in the fuel, there is a fear of poisoning of a catalyst used. Furthermore, aromatic compounds are contained in the crude oil-derived kerosene-gas oil distillate fractions, and there are many reports that particulate matter (PM) and nitrogen oxides (Nox) increase when the aromatic content is large. Accordingly, the fuel oil for diesel-powered vehicles is preferably one which is small in sulfur content and aromatic content.
On the other hand, in a synthetic hydrocarbon oil generated by the Fischer-Tropshch synthesis (hereinafter also referred to as the FT process) using a mixed gas comprising carbon monoxide and hydrogen, no sulfur compound is contained, because impurities in the mixed gas are removed. Further, paraffin is contained as a main component, so that the aromatic compounds are scarcely contained. Accordingly, the synthetic hydrocarbon oil by the FT process can be said to be a suitable fuel as a fuel for diesel-powered vehicles.
However, the synthetic fuel generated by the FT process comprises normal paraffin as a main component, but contains olefins and oxygen-containing compounds more or less. These substances are generally not contained in the crude oil-derived kerosene-gas oil distillate fractions. When the olefins are contained in large amounts in an automotive fuel, a peroxide is formed to possibly cause the occurrence of clogging in a fuel filter or the like. Further, even if the oxygen-containing compounds are only slightly contained, it contributes to corrosion of a fuel tank or a fuel supply system. Accordingly, in order to use the synthetic hydrocarbon oil generated by the FT process as the automotive fuel, it is necessary to remove the olefins and the oxygen-containing compounds.
It has hitherto been proposed to hydrotreat the synthetic hydrocarbon oil generated by the FT process, using a hydrogenation catalyst under conditions where isomerization and cracking do not occur, thereby removing the olefin and the oxygen-containing compound (for example, see patent document 1).    Patent Document 1: European Patent Application Publication No. 0583836